Up to now, a support for a photographic printing paper comprising a substrate coated on both sides thereof with a resin has been known. In such a support, titanium dioxide, a pigment, a blueing agent (including a blue pigment), and a fluorescent whitening agent are generally contained in a coating layer on an emulsion-coated side (U.S. Pat. No. 3,501,298).
Titanium dioxide used in such a support has an action to increase a light reflection coefficient as well as a waterproofing property. It is known that the larger the content of this titanium dioxide, the higher improvement in the image resolving power is obtained.
The waterproof resin layer is formed by melt extruding a waterproof resin containing titanium dioxide from a slit die in a short time. In the case where 20 weight % or more of titanium oxide is incorporated into a resin coating and a melt extrusion is carried out at a conventional extrusion temperature, i.e., 290.degree. to 350.degree. C., cracking of the waterproof resin layer (hereinafter referred to as film cracking) is generated or stripes are liable to form at a die lip portion of an extruder (hereinafter referred to as die lip stripes).
Generation of such film cracking not only damages the appearance of the product but also spoils the commercial value because of loss of a water proof property. Further, formation of the die lip stripes generates continuous stripes on a surface of a film or a laminate in a longitudinal direction; therefore, not only the appearance of the product is notably damaged but also unevenness is caused on a transparency of the film during a secondary processing such as stretching, which results in reduction of the commercial value.
In addition, in order to completely remove die lip stripes once formed, there is no way other than disassembling a die lip to rinse it, and a lot of labor and time is required for assembling and rinsing it, which results in a notable reduction of productivity. Accordingly, the resolution thereof is required.
Various methods for preventing formation of die lip stripes have so far been investigated. There are proposed, for example, from a viewpoint of an improvement in a resin composition, a method in which zinc oxide and a specific amount of a metal salt of higher fatty acid are used in combination (JP-A-53-102947; the term JP-A-as used herein means an unexamined published Japanese Patent No. Application), a method in which titanium dioxide dried reducing the weight thereof and a specific amount of a metal salt of fatty acid are used in combination (JP-A-57-16819), a method in which an anti-oxidation agent and a specific amount of a metal salt of fatty acid are used in combination (JP-A-60-11841), and a method in which the conditions titanium dioxide dried reducing the weight thereof, a metal salt of fatty acid and a specific melt viscosity of the composition are selected and used in combination (JP-B-59-42296). Further, many proposals have been made, such as a method in which, from a viewpoint that since a significant amount of staining and spots generate as the amount of titanium dioxide increases, titanium dioxide is required to be modified, a surface of titanium dioxide is treated with aluminum hydroxide hydrate (JP-A-57-108849 corresponding to U.S. Pat. No. 4,447,524), a method in which a treatment with di- to tetrahydrate alcohol is carried out (JP-A-58-17433 corresponding to U.S. Pat. No. 4,442,200), a method in which a treatment with alkyltitanate is carried out (JP-A-57-151942), a method in which a treatment with organic aluminum is carried out (JP-A-62-141544), a method in which a loss of the amount on drying of titanium dioxide is controlled to 0.35% or less (JP-A-59-1544, JP-A-59-121329, and JP-A-59-215234), and a method in which the conductivity of a suspension of titanium dioxide is controlled to 60 mbo/cm or less (JP-A-58-220140).
However, they are insufficient as a method for preventing formation of the die lip stripes.
In the case where an extrusion temperature is set to less than 290.degree. C. in order to improve such defects, the adhering power between a substrate such as a paper substrate and a waterproof resin markedly lowers.
Accordingly, a method in which the content of titanium dioxide is suppressed to 20 weight % or less even at a sacrifice of a resolving power has so far been employed.
Recently, a support for a high resolution printing paper has been developed in which an adhesion-providing resin is added to a titanium dioxide-containing layer and melt-extrusion is carried out at an extrusion temperature of 175.degree. to 290.degree. C. to increase the content of titanium dioxide in the layer (International Publication No. WO92/17538).
The detailed investigations on the above-described support for a high resolution printing paper made by the present inventors have clarified that not only is the dispersibility of titanium dioxide not sufficient, but separation of the titanium dioxide-containing layer from a cooling roll after extrusion-laminating is inferior, which results in a bad appearance of the product.
The extensive investigations made by the present inventors in order to more securely manufacture the support for the high resolution printing paper have resulted in finding that in the case where a waterproof resin layer containing a specific material is first provided and the above adhesion-providing resin and/or adhesive resin is not incorporated into a titanium dioxide-containing layer, titanium dioxide can readily be incorporated in an amount of 20 weight % or more and further that even in the case where extrusion molding is carried out at a melting temperature of less than 290.degree. C., the adhesive property is sufficiently maintained, and the present invention was reached.